I recently replaced the thermostat/housing on my 2009 R55 Clubman S. The initial trouble code of P0128(Coolant Temperature Below Thermostat Regulating Temperature)indicated that the thermostat was stuck open, and engine temps verified this on my Scan Gauge. After replacing the thermostat/housing I performed another OBDII scan which resulted in P0599, P0128, and P0117 trouble codes. Initial research led me to believe that a connection was not properly seated or a possible mismatch had occurred. However, after rechecking numerous times I discovered that it was not possible to switch wires as the connectors only will attach to the appropriate sensor or sending unit. Several individual suggested a faulty wiring harness may be the cause. I inspected the wiring and found no obvious burns, breaks, or loose connections. Stumped! I am going to try a new temperature sensor next as the cost is $40.00 versus $400.00 for a new harness. Any suggestions or insight would be appreciated.

 

Try finding the other end of the wires that plug into the t-stat. Then check resistance between both ends. That'll tell you if the wires are OK. Should show very little resistance, on the order of 0.1 ohms or less. If you get infinite resistance, the wire is not connected between those two ends.

If you get something in the middle, you've got a high-resistance connection somewhere.